U.S. patent application number 11/268692 was filed with the patent office on 2007-05-10 for method and apparatus of providing power to ignite and sustain a plasma in a reactive gas generator.
Invention is credited to Thomas Alexander, Ilya Bystryak, Madhuwanti Joshi, Alan Millner, Ken Tran.
Application Number | 20070103092 11/268692 |
Document ID | / |
Family ID | 37684469 |
Filed Date | 2007-05-10 |
United States Patent
Application |
20070103092 |
Kind Code |
A1 |
Millner; Alan ; et
al. |
May 10, 2007 |
Method and apparatus of providing power to ignite and sustain a
plasma in a reactive gas generator
Abstract
According to a first aspect, a power supply and a method of
providing power for igniting a plasma in a reactive gas generator
is provided that includes (i) coupling a series resonant circuit
that comprises a resonant inductor and a resonant capacitor between
a switching power source and a transformer, the transformer having
a transformer primary and a plasma secondary; (ii) providing a
substantially resonant AC voltage from the resonant capacitor
across the transformer primary, thereby inducing a substantially
resonant current within the transformer primary to generate the
plasma secondary; and (iii) upon generation of the plasma
secondary, the resonant inductor limiting current flowing to the
switching power supply. According to another aspect, bipolar high
voltage ignition electrodes can be used in conjunction with
inductive energy coupling to aid in plasma ignition.
Inventors: |
Millner; Alan; (Lexington,
MA) ; Alexander; Thomas; (Andover, MA) ;
Bystryak; Ilya; (Salem, MA) ; Tran; Ken;
(North Chelmsford, MA) ; Joshi; Madhuwanti;
(Burlington, MA) |
Correspondence
Address: |
PROSKAUER ROSE LLP
ONE INTERNATIONAL PLACE 14TH FL
BOSTON
MA
02110
US
|
Family ID: |
37684469 |
Appl. No.: |
11/268692 |
Filed: |
November 7, 2005 |
Current U.S.
Class: |
315/276 |
Current CPC
Class: |
H01J 37/32137 20130101;
H05H 1/46 20130101; H01J 37/32174 20130101; H01J 37/321
20130101 |
Class at
Publication: |
315/276 |
International
Class: |
H05B 41/16 20060101
H05B041/16 |
Claims
1. A method of providing power for igniting a plasma in a reactive
gas generator, comprising: coupling a series resonant circuit that
comprises a resonant inductor and a resonant capacitor between a
switching power source and a transformer, the transformer having a
transformer primary and a plasma secondary; providing a
substantially resonant AC voltage from the resonant capacitor
across the transformer primary, thereby inducing a substantially
resonant current within the transformer primary to generate the
plasma secondary; and upon generation of the plasma secondary, the
resonant inductor limiting current flowing to the switching power
supply.
2. The method of claim 1 further comprising: providing a first AC
voltage from the switching power source across the series resonant
circuit, the first AC voltage having a frequency at a substantially
resonant frequency of the series resonant circuit; in response to
the first AC voltage, providing the substantially resonant AC
voltage from the resonant capacitor across the transformer primary,
thereby inducing the substantially resonant current within the
transformer primary to generate the plasma secondary; modulating
the frequency of the first AC voltage from the switching power
source to a frequency greater than the substantially resonant
frequency of the series resonant circuit to further limit the
current flowing to the switching power supply subsequent to
generation of the plasma secondary.
3. The method of claim 1 further comprising: providing the series
resonant circuit including one or more resonant inductors in series
with the resonant capacitor, the resonant capacitor coupled in
parallel to the transformer primary; limiting the current flowing
to the switching power supply through the one or more inductors
upon generation of the plasma secondary.
4. The method of claim 3 further comprising: providing the series
resonant circuit further including a second capacitor being coupled
in series between the one or more resonant inductors and the
resonant capacitor or between the resonant capacitor and the
transformer, the second capacitor being a DC blocking capacitor or
a second resonant capacitor.
5. The method of claim 3 further comprising: providing a first AC
voltage from the switching power source across the series resonant
circuit, the first AC voltage having a frequency at a substantially
resonant frequency of the series resonant circuit; in response to
the first AC voltage, providing the substantially resonant AC
voltage from the resonant capacitor across the transformer primary,
thereby inducing the substantially resonant current within the
transformer primary to generate the plasma secondary; modulating
the frequency of the first AC voltage from the switching power
source to a frequency greater than the substantially resonant
frequency of the series resonant circuit to further limit the
current flowing through the one or more inductors to the switching
power supply subsequent to generation of the plasma secondary.
6. The method of claim 1 wherein the switching power source is a
half bridge inverter.
7. The method of claim 1 wherein the switching power source is a
full bridge inverter.
8. The method of claim 1 further comprising: coupling a second
transformer primary to the transformer, the second transformer
primary having a center tap or a substantially center tap;
grounding the center tap of the second transformer primary to
provide a first lead and a second lead of the second transformer
primary; coupling the first lead to a first ignition electrode
positioned at a first location about the plasma secondary and
coupling the second lead to a second ignition electrode opposing
the first ignition electrode at a second location about the plasma
secondary; and applying a voltage of a first polarity to the first
lead and a voltage of a second polarity to the second lead,
resulting in electric field flux traversing a cross sectional area
between the first and second electrode to generate the plasma
secondary.
9. The method of claim 8 further comprising: providing a plasma
chamber for containing the plasma secondary; and coupling the first
lead to the first ignition electrode on an outer surface of the
plasma chamber and coupling the second lead to the second ignition
electrode on the outer surface of the plasma chamber opposing the
first ignition electrode; and applying a voltage of a first
polarity to the first lead and a voltage of a second polarity to
the second lead, resulting in the electric field flux traversing a
cross sectional area of the plasma chamber between the first and
second electrode to generate the plasma secondary.
10. The method of claim 1 further comprising: providing a plasma
chamber for containing the plasma secondary; and coupling flux from
the transformer primary to the plasma secondary through a magnetic
core surrounding a portion of the plasma chamber and the
transformer primary; flowing an inlet gas through the plasma
secondary to convert the inlet gas into a reactive gas.
11. A power supply for igniting a plasma in reactive gas generator,
comprising: a switching power source; a transformer comprising a
transformer primary and a plasma secondary; and a series resonant
circuit comprising a resonant inductor and a resonant capacitor
coupled between the switching power source and the transformer
primary, the resonant capacitor providing a substantially resonant
AC voltage across the transformer primary, thereby inducing a
substantially resonant current within the transformer primary to
generate the plasma secondary; upon generation of the plasma
secondary, the resonant inductor limiting current flowing to the
switching power supply.
12. The power supply of claim 11 further comprising: a controller;
the switching power source providing a first AC voltage across the
series resonant circuit, the first AC voltage having a frequency at
a substantially resonant frequency of the series resonant circuit;
in response to the first AC voltage, the resonant capacitor
providing the substantially resonant AC voltage from across the
transformer primary, thereby inducing the substantially resonant
current within the transformer primary to ignite the plasma
secondary; the controller providing signals to the switching power
source to modulate the frequency of the first AC voltage to a
frequency greater than the substantially resonant frequency of the
series resonant circuit to further limit the current flowing to the
switching power supply subsequent to generation of the plasma
secondary.
13. The power supply of claim 11 wherein the series resonant
circuit comprises: one or more resonant inductors in series with
the resonant capacitor, the resonant capacitor coupled in parallel
to the transformer primary; the one or more inductors limiting the
current flowing to the switching power supply upon generation of
the plasma secondary.
14. The power supply of claim 13 wherein the series resonant
circuit further comprises: a second capacitor being coupled in
series between the one or more resonant inductors and the resonant
capacitor or between the resonant capacitor and the transformer,
the second capacitor being a DC blocking capacitor or a second
resonant capacitor.
15. The power supply of claim 13 further comprising: a controller;
the switching power source providing a first AC voltage across the
series resonant circuit, the first AC voltage having a frequency at
a substantially resonant frequency of the series resonant circuit;
in response to the first AC voltage, the resonant capacitor
providing the substantially resonant AC voltage across the
transformer primary, thereby inducing the substantially resonant
current within the transformer primary to generate the plasma
secondary; the controller modulating the frequency of the first AC
voltage from the switching power source to a frequency greater than
the substantially resonant frequency of the series resonant circuit
to further limit the current flowing through the one or more
inductors to the switching power supply subsequent to generation of
the plasma secondary.
16. The power supply of claim 11 wherein the switching power source
comprises a half bridge inverter.
17. The power supply of claim 11 wherein the switching power source
comprises a full bridge inverter.
18. The power supply of claim 11 further comprising: a second
transformer primary coupled to the transformer, the second
transformer primary having a center tap or a substantially center
tap being grounded to provide a first lead and a second lead of the
second transformer primary; a first ignition electrode positioned
at a first location about the plasma secondary, the first ignition
electrode being coupled to the first lead; and a second ignition
electrode opposing the first ignition electrode at a second
location about the plasma secondary, the second ignition electrode
being coupled to the second lead; the second transformer primary
applying a voltage of a first polarity to the first lead and a
voltage of a second polarity to the second lead, resulting in
electric field flux traversing a cross sectional area between the
first and second electrode to generate the plasma secondary.
19. The power supply of claim 18 further comprising: a plasma
chamber containing the plasma secondary; the first lead being
coupled to the first ignition electrode on an outer surface of the
plasma chamber and the second lead being coupled to the second
ignition electrode on the outer surface of the plasma chamber
opposing the first ignition electrode; the second transformer
primary applying the voltage of a first polarity to the first lead
and the voltage of the second polarity to the second lead,
resulting in the electric field flux traversing a cross sectional
area of the plasma chamber between the first and second electrode
to generate the plasma secondary.
20. The power supply of claim 11 wherein the plasma secondary is
contained within a plasma chamber.
21. The power supply of claim 20 further comprising: a magnetic
core surrounding a portion of the plasma chamber and the
transformer primary for coupling flux from the transformer primary
to the plasma secondary.
22. A method of providing power for igniting a plasma in a reactive
gas generator, comprising: providing a transformer having a
transformer primary, the transformer primary having a center tap or
a substantially center tap; grounding the center tap of the
transformer primary to provide a first lead and a second lead of
the transformer primary; coupling the first lead to a first
ignition electrode positioned at a first location about a plasma
body and coupling the second lead to a second ignition electrode
opposing the first ignition electrode at a second location about
the plasma body; and applying a voltage of a first polarity to the
first lead and a voltage of a second polarity to the second lead,
resulting in electric field flux traversing a cross sectional area
between the first and second electrode to generate the plasma
body.
23. The method of claim 22 further comprising: providing a plasma
chamber for containing the plasma secondary; coupling the first
lead to the first ignition electrode on an outer surface of the
plasma chamber and coupling the second lead to the second ignition
electrode on the outer surface of the plasma chamber opposing the
first ignition electrode; and applying the voltage of the first
polarity to the first lead and the voltage of the second polarity
to the second lead, resulting in electric field flux traversing a
cross sectional area of the plasma chamber between the first and
second electrode to generate the plasma body.
24. The power supply of claim 11 further comprising: a controller;
the switching power source providing a first AC voltage across the
series resonant circuit, the first AC voltage having a frequency at
a substantially resonant frequency of the series resonant circuit;
in response to the first AC voltage, the resonant capacitor
providing the substantially resonant AC voltage from across the
transformer primary, thereby inducing the substantially resonant
current within the transformer primary to ignite the plasma
secondary; the controller providing signals to the switching power
source to modulate the duty cycle of the first AC voltage to
further limit the current flowing to the switching power supply
subsequent to generation of the plasma secondary.
25. The method of claim 1 further comprising: providing a first AC
voltage from the switching power source across the series resonant
circuit, the first AC voltage having a frequency at a substantially
resonant frequency of the series resonant circuit; in response to
the first AC voltage, providing the substantially resonant AC
voltage from the resonant capacitor across the transformer primary,
thereby inducing the substantially resonant current within the
transformer primary to generate the plasma secondary; modulating
the duty cycle of the first AC voltage from the switching power
source to further limit the current flowing to the switching power
supply subsequent to generation of the plasma secondary.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to the field of generating
reactive gas containing ions, free radicals, atoms and molecules
and to apparatus for and methods of providing power for igniting a
plasma in a reactive gas generator.
BACKGROUND OF THE INVENTION
[0002] Plasma discharges can be used to excite gases to produce
reactive gas containing ions, free radicals, atoms and molecules.
Reactive gases are used for numerous industrial and scientific
applications including processing solid materials such as
semiconductor wafers, powders, and other gases.
[0003] One example of a reactive gas is atomic fluorine, which can
be used to clean chemical vapor deposition (CVD) chambers for
deposition of thin films onto substrate surfaces. CVD chambers need
to be routinely cleaned to remove the deposits that build up on the
surfaces of chamber parts other than the substrate surfaces. Wet
cleaning of a chamber is labor intensive and hazardous to workers,
while cleaning the chamber with atomic fluorine generated by a
plasma source allows the deposits to be removed without opening the
chamber to atmosphere, improving tool productivity and working
conditions. Typical source gases for atomic fluorine include
perfluorocompounds (PFCs) such as NF.sub.3, CF.sub.4, CHF.sub.3,
C.sub.2F.sub.6, and C.sub.4F.sub.8.
[0004] Another example of a reactive gas is atomic oxygen, which
can be used for photoresist removal in microelectronics
fabrication. After pattern generation, photoresist is removed by
exposing the wafer surface to atomic oxygen generated by a plasma
source. Atomic oxygen reacts rapidly and selectively with
photoresist, allowing the process to be conducted in a vacuum and
at relatively low temperature.
SUMMARY OF THE INVENTION
[0005] Plasma can be generated through inductive coupling of energy
from a power supply into a gas that is capable of being transformed
into a plasma. Known techniques for providing power to ignite and
sustain a plasma include circuitry to prevent damage to power
supply semiconductor devices upon plasma ignition. However, such
techniques are generally costly and are not reliable.
[0006] The invention features methods and power supplies that
provide power to ignite and sustain a plasma in a reactive gas
generator. Advantages of particular embodiments include prevention
of damage to power supply semiconductor devices at reduced cost and
increased reliability and performance.
[0007] According to a first aspect of the invention, a power supply
and a method of providing power for igniting a plasma in a reactive
gas generator is provided that includes (i) coupling a series
resonant circuit that comprises a resonant inductor and a resonant
capacitor between a switching power source and a transformer having
a transformer primary and a plasma secondary; (ii) providing a
substantially resonant AC voltage from the resonant capacitor
across the transformer primary, thereby inducing a substantially
resonant current within the transformer primary to generate the
plasma secondary; and (iii) upon generation of the plasma
secondary, the resonant inductor limiting current flowing to the
switching power supply.
[0008] Particular embodiments can further include providing a first
AC voltage from the switching power source across the series
resonant circuit, the first AC voltage having a frequency at a
substantially resonant frequency of the series resonant circuit; in
response to the first AC voltage, providing the substantially
resonant AC voltage from the resonant capacitor across the
transformer primary, thereby inducing the substantially resonant
current within the transformer primary to generate the plasma
secondary; and modulating the frequency of the first AC voltage
from the switching power source to a frequency greater than the
substantially resonant frequency of the series resonant circuit to
further limit the current flowing to the switching power supply
subsequent to generation of the plasma secondary.
[0009] Particular embodiments can include providing the series
resonant circuit with one or more resonant inductors in series with
the resonant capacitor, the resonant capacitor coupled in parallel
to the transformer primary; and limiting the current flowing to the
switching power supply through the one or more inductors upon
generation of the plasma secondary. In such embodiments, the method
can include providing a first AC voltage from the switching power
source across the series resonant circuit, the first AC voltage
having a frequency at a substantially resonant frequency of the
series resonant circuit; in response to the first AC voltage,
providing the substantially resonant AC voltage from the resonant
capacitor across the transformer primary, thereby inducing the
substantially resonant current within the transformer primary to
generate the plasma secondary; and modulating the frequency of the
first AC voltage from the switching power source to a frequency
greater than the substantially resonant frequency of the series
resonant circuit to further limit the current flowing through the
one or more inductors to the switching power supply subsequent to
generation of the plasma secondary.
[0010] Particular embodiments can also include providing the series
resonant circuit further including a second capacitor being coupled
in series between the one or more resonant inductors and the
resonant capacitor or between the resonant capacitor and the
transformer, the second capacitor being a DC blocking capacitor or
a second resonant capacitor.
[0011] Particular embodiments can further include the steps of
providing a plasma chamber for containing the plasma secondary;
coupling flux from the transformer primary to the plasma secondary
through a magnetic core surrounding a portion of the plasma chamber
and the transformer primary; and flowing an inlet gas through the
plasma secondary to convert the inlet gas into a reactive gas.
[0012] According to a second aspect of the invention, a power
supply and a method of providing power for igniting a plasma in a
reactive gas generator are provided that include (i) providing a
transformer having a transformer primary, the transformer primary
having a center tap or a substantially center tap; (ii) grounding
the center tap of the transformer primary to provide a first lead
and a second lead of the transformer primary; (iii) coupling the
first lead to a first ignition electrode positioned at a first
location about a plasma body and coupling the second lead to a
second ignition electrode opposing the first ignition electrode at
a second location about the plasma body; and (iv) applying a
voltage of a first polarity to the first lead and a voltage of a
second polarity to the second lead, resulting in electric field
flux traversing a cross sectional area between the first and second
electrode to generate the plasma body.
[0013] Particular embodiments can further include (v) providing a
plasma chamber for containing the plasma secondary; (vi) coupling
the first lead to the first ignition electrode on an outer surface
of the plasma chamber and coupling the second lead to the second
ignition electrode on the outer surface of the plasma chamber
opposing the first ignition electrode; and (vii) applying the
voltage of the first polarity to the first lead and the voltage of
the second polarity to the second lead, resulting in electric field
flux traversing a cross sectional area of the plasma chamber
between the first and second electrode to generate the plasma
body.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0015] FIG. 1 is a diagram illustrating a reactive gas generator to
which embodiments of the invention may be applied.
[0016] FIGS. 2A and 2B are circuit diagrams illustrating known
power supply configurations that provide power to ignite and
sustain a plasma.
[0017] FIG. 3A is a circuit diagram illustrating a power supply
that provides power to ignite and sustain a plasma in a reactive
gas generator according to one embodiment.
[0018] FIG. 3B is a circuit diagram illustrating a power supply
that provides power to ignite and sustain a plasma in a reactive
gas generator according to another embodiment.
[0019] FIGS. 4A and 4B are diagrams that illustrate a high voltage
ignition controller and an arrangement of corresponding ignition
electrodes according to the prior art.
[0020] FIGS. 5A and 5B are diagrams that illustrate a high voltage
ignition controller and an arrangement of corresponding ignition
electrodes according to one embodiment.
[0021] FIGS. 6A and 6B are a schematic diagrams illustrating a
particular arrangement of ignition electrodes about a plasma
chamber.
DETAILED DESCRIPTION
[0022] The invention features methods and power supplies that
provide power to ignite and sustain a plasma in a reactive gas
generator. Advantages of particular embodiments include prevention
of damage to power supply semiconductor devices at reduced cost and
increased reliability and performance.
[0023] FIG. 1 is a diagram illustrating a reactive gas generator to
which embodiments of the invention may be applied. As illustrated,
the reactive gas generator 1 includes a power supply 10 and a
plasma chamber 20. The plasma chamber 20 includes an inlet 40 for
receiving a gas (e.g., Argon) for transformation into a plasma
(e.g., Ar+). Once generated, the plasma may be used directly, or
can be used to excite one or more other source gases into
corresponding reactive gases that exit the generator at outlet 42
and into, for example, a process chamber 45.
[0024] In order to ignite the plasma, the power supply 10 includes
a transformer 30. The transformer primary includes a primary
winding 32 wrapped about a portion of a magnetic core 34. Energy
from the power supply 10 is inductively coupled via the transformer
primary to the gas traveling through the chamber 20 to ignite, or
generate, the plasma. The ignited plasma within the plasma chamber
20 serves as the transformer secondary. Specifically, the power
supply 10 applies an excitation voltage of high magnitude across
the primary winding 32 of the transformer. This high excitation
voltage induces high voltage current in the winding 32, thereby
generating an alternating magnetic field through the magnetic core
34 across a path of the gas. As a result, current is induced within
the gas, causing its ignition into a plasma. Once the plasma is
generated, the plasma can be used to excite other source gas,
producing the desired reactive gas for specific applications.
[0025] FIGS. 2A and 2B are circuit diagrams representing known
power supply configurations that provide power to ignite and
sustain a plasma. The plasma secondary 36 of the transformer can be
represented as an equivalent circuit having an inductance L and a
reactance Z. In both configurations, the power supply includes a
series resonant circuit 50 to provide the high excitation voltage
to the primary winding 32 of the transformer. Specifically, the
series resonant circuit 50 is formed using the primary winding 32
as a resonant inductor and coupling a resonant capacitor 54 in
series between a switching power source 60 and the primary winding
32. In response to an excitation voltage Vs from the switching
power source 60 having a frequency at or near the resonant
frequency of the resonant circuit 50, a resonant voltage or a
substantially resonant voltage V.sub.RES is applied across the
primary winding 32 of the transformer 30 inducing a high voltage,
resonant current I.sub.RES in the primary winding 32 for triggering
ignition of the plasma as previously described.
[0026] Prior to plasma ignition, the inductance of primary winding
32 in FIGS. 2A and 2B limits the high voltage, resonant current
I.sub.RES to a safe operating level for return to the switching
power source 60. However, once the plasma ignites, the inductance
of the primary winding 32 is reduced (e.g., effectively shorted),
enabling additional current to flow to the switching power source
60. Also, due to the reduced inductance, the load at this operating
frequency becomes capacitive, which is known by those familiar with
this art to cause stressful hard transitions in the AC source. Both
the high current and the hard transitions can be a potential hazard
causing damage to constituent semiconductor devices of the power
source (e.g., FETs, MOSFETs, IGBTs and the like). A short circuit
current, as used herein, refers to any increase in current flowing
through the primary winding as a result of the reduced inductance
of the winding upon ignition of the plasma.
[0027] In order to limit the high circuit current after plasma
ignition, the circuit of FIG. 2A includes a controllable relay 56
bridged across the resonant capacitor 54. In operation, the relay
56 is open during ignition of the plasma to couple the resonant
capacitor 54 to the circuit and closed after plasma ignition to
decouple the capacitor 54 from the circuit. By decoupling the
capacitor 54 from the circuit, the resonant voltage V.sub.RES drops
to the source excitation voltage V.sub.s across the primary winding
32 resulting in a reduction in the current induced in the primary
winding and the current flowing back to the power source 60. Also,
the load is no longer capacitive, since the relay removes the
capacitor from the active circuit. Thus, the likelihood of
semiconductor damage within with switching power source is
reduced.
[0028] Disadvantages of this power supply configuration, however,
include the high cost associated with the circuit due to the need
for the relay 56 and the time delays or potential failures of the
relay 56 and plasma ignition detection circuitry (not shown). If
detection of plasma ignition fails or is delayed, short circuit
current I.sub.RES+ can return to the power source causing damage.
This circuit requires very fast response of the electronics to
avoid destructive damage to the power semiconductors providing the
excitation voltage.
[0029] With respect to FIG. 2B, the high circuit current I.sub.RES+
is limited using frequency modulation of the source excitation
voltage. Specifically, the switching power source 60 utilizes
frequency modulation to provide excitation voltages V.sub.s of
varying frequency in order to control the current returning to the
switching power source. Prior to plasma ignition, the switching
power source 60 provides a source excitation voltage V.sub.s having
a frequency at or near the resonant frequency of the series
resonant circuit 50. Once plasma ignition is detected, a frequency
modulation controller 80 adjusts the frequency of the excitation
voltage V.sub.s away from the resonant frequency, causing a
reduction in the magnitude of the voltage V.sub.RES applied to the
primary winding 32 and thus a decrease in the current returning to
the switching power source 60.
[0030] By controlling the frequency of the source excitation
voltage, this circuit can control the current through the primary
winding 32 without the need for a relay. However, like the circuit
of FIG. 2A, semiconductor damage can still occur if detection of
plasma ignition fails or is delayed causing a high circuit current
I.sub.RES+ to return to the power source. The frequency range
required for safe operation may exceed the capabilities of the
circuit elements. This circuit also requires very fast response of
the electronics to avoid destructive damage to the power
semiconductors providing the excitation voltage.
[0031] The present invention features methods and power supplies
that provide power to ignite and sustain a plasma in a reactive gas
generator. Advantages of particular embodiments include prevention
of damage to power supply semiconductor devices at reduced cost and
increased reliability and performance.
[0032] FIG. 3A is a circuit diagram illustrating a power supply
that provides power to ignite and sustain a plasma in a reactive
gas generator. In this embodiment, the power supply includes a
switching power source 60, a transformer 30 comprising a primary
winding 32, magnetic core 34, and a plasma secondary 36. A series
resonant circuit 100 is driven by the switching power source 60 and
is coupled with its capacitor in parallel with the switching power
source 60 and the transformer 30. The series resonant circuit
includes an LC circuit having a resonant inductor 110 in series
with a resonant capacitor 120. The resonant capacitor 120 in turn
is coupled in parallel to the primary winding 32. The resonant
inductor 110 and resonant capacitor 120 form a low pass filter
network.
[0033] In operation, the switching power source 60 can be a half
bridge or full bridge power converter as known in the art or as
described in U.S. patent application Ser. No. 11/077,555, entitled
"Control Circuit for Switching Power Supply," the entire contents
of which are incorporated herein by reference. The power source 60
provides an AC excitation voltage V.sub.s having a frequency at or
substantially at the resonant frequency of the series resonant
circuit. In one embodiment, the frequency of the excitation voltage
is above the resonant frequency of the resonating inductance and
capacitance. For higher current and power, the frequency is closer
to the resonance. For lower current and power, the frequency is
farther from it. Typically, the frequency shifts by less than 50%
of the resonant frequency over the required range. For a Q factor
of 10 for the circuit with no ignition, the current shifts a small
fraction (much less than 10%) of the maximum value. In practice,
the commanded value of resonant current during ignition is
preferably 1.5 to 3 times the value needed for steady state plasma
operation.
[0034] The excitation voltage V.sub.s is applied across the series
resonant circuit 100, causing the resonant circuit to provide a
substantially resonant AC voltage V.sub.RES across the transformer
primary, inducing a substantially resonant current I.sub.RES within
the transformer primary to ignite the plasma.
[0035] The generated plasma 36 serves as the secondary of
transformer 30. The plasma secondary can be represented with an
equivalent circuit having an inductance L and a reactance Z.
However, it should be understood that the plasma secondary 36 is
actually a body of plasma traversing a volume within a plasma
chamber. The plasma chamber 20 can be manufactured in the shape of
a toroid or other shapes that provide a toroidal flow of the
gas.
[0036] Once the plasma ignites, a short circuit current does not
flow to the switching power source 60. Rather, although the
inductance of the primary winding 32 of the transformer 30 is
reduced after plasma ignition, the resonant inductor 110 of the
series resonant circuit 100 continues to serve as a low pass filter
limiting current through the primary winding 32 to a safe operating
level for return to the switching power supply 60. Also, the effect
of the reduced plasma impedance reflected through the transformer
across the capacitor 120 results in an inductive load, avoiding the
stressful hard transitions. Thus, damage to the constituent
semiconductor devices in the power source 60 due to high current is
substantially prevented. Advantages of this embodiment include
reduced cost due to the lack of a relay component and increased
reliability due to the circuit's ability to limit current
regardless of whether plasma ignition is detected.
[0037] FIG. 3B is a circuit diagram illustrating a power supply
that provides power to ignite and sustain a plasma in a reactive
gas generator according to another embodiment. In this illustrated
diagram, the power supply is similar to that of FIG. 3A except that
the circuit includes one or more additional capacitors 122 in
series with the resonant capacitor 120. These additional
capacitors, which may be placed either between the inductor and the
original resonant capacitor or between the original resonant
capacitor and the transformer, may serve as DC blocking capacitors
or additional resonant capacitors. The circuit also includes an
additional resonant inductor 112 coupled in series with the
resonant capacitor 120 to assist during plasma ignition and to
limit current flowing back to the power source after plasma
ignition.
[0038] According to another embodiment, the power supplies of FIGS.
3A and 3B can include a controller 140 that can be used to further
control the flow of current and power into the plasma gas in
response to detection of plasma ignition. In the illustrated
embodiment, the controller 140 provides signals to the switching
power source 60 that can cause the power source to modulate the
frequency and/or duty cycle (e.g., pulse width modulation or PWM)
of the AC excitation voltage V.sub.s, resulting in a desired
current flowing within the power supply circuit. The desired
current can be configured by setting a reference current REF that
is input to the controller 140. The controller 140 (i) senses the
current at one or more predetermined positions (e.g., positions A,
B) in the circuit (ii) compares it with the reference current, and
(iii) sends control signals to the switching power source 60 to
make adjustments to the operating frequency or duty cycle of the
power source using known frequency modulation or pulse width
modulation techniques.
[0039] For example, once the gas ignites into a plasma, the
inductance of the transformer is reduced driving additional current
through the primary winding. This condition is detected by a
current sensor at points A or B which feeds back to the controller
140 to shift the frequency away from the resonant frequency to a
frequency for maintenance of the plasma flow and safe operation of
the power supply. The frequency is increased, reducing the amount
of current by the action of the resonant inductors. This provides a
means of regulating the current or power flow by sensing this
parameter anywhere in the circuit and adjusting the frequency to
achieve the desired level of excitation of the gas. Alternately or
in conjunction with frequency modulation, pulse width modulation
(PWM) can be used to adjust the duty cycle of the switching power
source to achieve a broader dynamic range of regulation. According
to a particular embodiment, low output current can be regulated
with pulse width modulation (PWM) while power can be regulated with
pulse frequency modulation (PFM), or vice versa. This combined use
of PFM and PWM results in a broader dynamic range of regulation for
the entire power conversion system. Another advantage of the
combined use of PFM and PWM is reduced reverse recovery stress on
the body diode of MOSFETs in the switching power source at low
current or power, as opposed to regulating with PFM alone in which
the modulating frequencies must approach the Megahertz range.
[0040] According to another aspect of the invention, bipolar high
voltage ignition electrodes can be used in conjunction with
inductive energy coupling to aid in plasma ignition. Preferably,
the ignition electrodes are arranged in or about the plasma chamber
for capacitively coupling energy to the gas flowing within the
chamber to ignite and sustain a plasma. The voltages impressed upon
the ignition electrodes to ignite and sustain a plasma are
typically controlled by a high voltage ignition controller. Once
the plasma is generated, the energy coupled through inductance, as
previously described in FIGS. 1-3B, is sufficient to maintain the
plasma state and the ignition electrodes can be disabled, or
otherwise "turned off."
[0041] FIGS. 4A and 4B are diagrams that illustrate a high voltage
ignition controller and an arrangement of corresponding ignition
electrodes according to the prior art. Specifically, the high
voltage ignition controller 200 includes a second transformer
primary in the form of a winding 210 wrapped about a portion of the
magnetic core 34 of the transformer 30. When the power supply 10
provides an excitation voltage across the primary winding 32, a
current is induced within the secondary primary winding 210
according to a turns ratio. A first lead 210a of the winding is
grounded, while the second lead is switchably connected to one or
more electrodes 230 arranged about or within the plasma chamber 30.
During plasma ignition, the relay 220 of the controller 200 is
closed so that the voltage from the second lead 210b can be applied
to the one or more ignition electrodes 230. Once the plasma
ignites, the relay 220 is opened, disabling the capacitive
discharge.
[0042] FIG. 4B illustrates a typical arrangement of ignition
electrodes positioned about a cross section of a plasma chamber or
channel within the chamber 250. Specifically, ignition electrodes
230a, 230b are positioned on opposing sides of the plasma chamber
250 and are switchably connected to the second lead of the
secondary primary winding 210b, resulting in both electrodes having
the same polarity. Ground electrodes 240a, 240b are also positioned
on opposing sides of the plasma chamber or channel 250 at an offset
between the ignition electrodes 230a, 230b.
[0043] When the high voltage ignition controller 200 directs the
relay 220 to connect the ignition electrodes, the voltage at the
second lead 210b is applied to both electrodes 230, generating a
spatially limited electric field between ignition electrodes 230
and ground electrodes 240 as shown. This limited electric field, or
fringe field, can fail to ignite a plasma under certain gas flow
and pressure conditions within the chamber, or can fail to
propagate the ignition into the bulk of the plasma. Thus, in those
instances, a stronger electric field is required in order to ignite
the plasma. Prior art techniques involve higher voltages being
applied to the ignition electrodes 230 to generate the requisite
electric flux, resulting in increased insulation and its related
cost.
[0044] A second aspect of the invention addresses this issue
through the use of a central ground tap that is connected to the
secondary primary winding. The use of a central ground tap results
in the leads that extend from the secondary primary winding being
bipolar and, thus, enables the generation of positive ignition
electrodes and negative ignition electrodes. By placing the
positive and negative ignition electrodes such that they oppose one
another, an increase in the amount of electric flux that can be
realized across the plasma chamber or channel. This results in less
voltage to ground being required to generate the requisite amount
of electric flux to ignite the plasma gas.
[0045] FIGS. 5A and 5B are diagrams that illustrate a high voltage
ignition controller and an arrangement of corresponding ignition
electrodes according to one embodiment. Specifically, the high
voltage ignition controller 300 includes a second transformer
primary in the form of a winding 310 wrapped about a portion of the
magnetic core 34 of the transformer 30. The second transformer
primary 310 includes a center tap or a substantially center tap
310c that is grounded to provide a first lead 310a having a first
polarity and a second lead 310b having a second polarity that is
different from the first.
[0046] For example, when the power supply 10 provides an excitation
voltage across the primary winding 32, a current is induced within
the secondary primary winding 310, such that a negative polarity
voltage -V can be applied to the first lead 310a and a positive
polarity voltage +V can be applied to the second lead 310b. Both
the first lead 310a and the second lead 310b are switchably
connected to a respective ignition electrode 330a, 330b through
relays 320a, 320b, if this is desired for example to extend
electrode life.
[0047] In this illustrated embodiment, the first lead 310a is
coupled to the first ignition electrode 330a on an outer surface of
the plasma chamber 350 and the second lead 310b is coupled to the
second ignition electrode 330b on the outer surface of the plasma
chamber 350 opposing the first ignition electrode 330a. Ground
electrodes 240a, 240b can also be positioned on opposing sides of
the plasma chamber or channel 300 at an offset between the ignition
electrodes 330a, 330b.
[0048] During plasma ignition, the relays 320a, 320b of the
controller 300 are closed so that the positive voltage of the first
lead 310a and the negative voltage of the second lead 310b are
applied to the respective ignition electrodes 330a, 330b, resulting
in a strong electric field flux traversing a cross sectional area
of the plasma chamber or channel 350 between the first and second
electrodes to generate the plasma secondary. Once the plasma
ignites, the relays 320a, 320b can be opened, disabling the
capacitive discharge.
[0049] FIGS. 6A and 6B are schematic diagrams illustrating a
particular arrangement of ignition electrodes about a plasma
chamber. FIG. 6A represents a view of an outer surface 410 of a
plasma chamber having ignition electrodes 415a, 415b, 415c, and
415d. Each electrode is associated with a positive or negative
polarity. Referring to FIG. 6B, a three dimensional view of a
plasma chamber is shown with the outer surface 410 of FIG. 6A
having an opposing outer surface 420. FIG. 6B illustrates that the
opposing surface 420 of the chamber 450 includes ignition
electrodes 425a, 425b, 425c and 425d having polarities that
directly oppose the electrode polarities of surface. According to
such configurations, a strong electric field flux can be generated
within the plasma chamber between the bipolar electrodes on the
opposing surfaces 410, 420 during plasma ignition. As a result, the
requisite amount of electric flux to ignite the plasma gas can be
realized without a corresponding increase in applied voltage to the
ignition electrodes.
[0050] Although embodiments of the invention have been described
with respect to exemplary circuit diagrams, the invention is not so
limited. Furthermore, other embodiments of the invention may
include alternative implementation details, including the use of
hybrid circuits, discrete circuits, MOSFETs, IGBTs and others known
to those skilled in the art.
[0051] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *